Tuning in to April meteor showers

Amateur astronomers capture radio echoes from fiery meteors in April 99

Apr. 27, 1999: When the Lyrid meteor shower reached its peak on April 22, much of North America was overcast, frustrating stargazers who hoped to view the first major meteor shower of the 1999 observing season. Michael Boschat, an amateur astronomer from Halifax, Nova Scotia, was clouded out, too, but he observed over 150 meteors -- by listening to his radio.

This year many amateur radio operators tuned into the Lyrids using a technique called radio forward scattering. When fast-moving meteoroids strike Earth's atmosphere they heat and ionize the air in their path. The luminous ionized trails are not only visually striking -- they also reflect radio waves. During a major meteor shower, radio signals from TV stations, RADAR facilities, and AM/FM transmitters are constantly bouncing off short lived meteor trails. For those who know how to listen, it's easy to hear the echoes.

Right: This photo of a Geminid meteor streaking through the Big Dipper was captured by Yukihiro Kida in Hamada, Japan at 16:14 UT on Dec.13, 1998. The colorful tail consists of ionized air that can reflect radio waves from TV, radar, and AM/FM radio transmitters.

Boschat, who monitored an array of Channel 6 TV transmitters at 83.25 MHz, counted a maximum of 46 detections per hour at 11 UT on April 22, 1999, compared to visual spotters around the world who observed fewer than 5-10 Lyrids per hour on the same evening. It was a meager year for visual Lyrid meteors, but radio Lyrids were relatively plentiful.

Left: Artist's concept of radio meteor observing. Meteor trains reflect transmissions from stations that are over the horizon, and normally impossible to detect. Whenever a meteor passes by with the correct geometry, listeners hear a brief "ping" on the receiver's loudspeaker. more information from the North American Meteor Network.

One of the advantages of radio observing is that meteors can be detected when skies are cloudy, or even during daylight. In fact, the International Meteor Organization lists a dozen "daylight meteor showers" that peak after sunrise and are monitored almost exclusively by radio observations. The most intense of these is the Arietids which peak each year around June 8. They usually produce 60 to 100 radio detections per hour.

Radio observing has some advantages at night, too. The human eye can only see shooting stars brighter than 6th magnitude, but radio methods can detect meteors that are at least 5 times dimmer. That's the main reason radio observers detected more Lyrids than their visual counterparts.

A disadvantage of radio observations for serious meteor watchers is that a ping heard in the loudspeaker doesn't tell the listener where the meteor came from. Was it a Lyrid, emanating from a radiant in the constellation Lyra, or a sporadic, a member of the random background of shooting stars that may come from any direction in the sky? Visual observers can discriminate easily, but radio observers cannot. Nevertheless, when skies are cloudy radio observing can't be beat.

from a sporadic meteor on April 11, 1999. He used an ICOM R8500 communications receiver to monitor 217 MHz transmissions from the Navy Space Surveillance Radar located in Lake Kickapoo, TX. He was observing about one hour after sunset in support of the NASA Meteor Balloon project, at a time when the meteor rate was expected to be low. Nevertheless, he was rewarded with a bright visual meteor that also registered on his radio setup. A frequency-time plot of Nelson's data shows that he detected one echo from ionized air around the fast-moving meteoroid followed by a second echo from the residual meteor train left behind after the meteoroid distintegrated.

Left: Stan Nelson's radio meteor echo looks like an "L" in this frequency-time plot. The slanted, nearly vertical piece is the reflection from ionized air around the rapidly-moving meteoroid (sometimes called the "head" of the meteor). The doppler-shifted frequency of the reflection changes rapidly because the meteoroid moves across the line-of-sight of the transmitter at high speed. After the meteoroid disintegrates, all that remains is a slowly moving trail of ionized air. The velocity of the residual trail is low, typically no more than 0.02 km/s, compared to 30 - 70 km/s for the meteoroid. The reflection from the slowly moving gas does not experience a significant doppler shift, so it appears as a horizontal line (the bottom of the "L") in the dynamic spectrum.

How it's Done

If you're interested in detecting radio meteors, the procedures are relatively simple. You'll need a good commercial radio receiver and an aerial. Although meteor trains can reflect radio waves at almost any frequency, the best frequencies to try are usually between 50 and 120 MHz. Many observers use a common FM radio tunable between 88 and 108 MHz and a Yagi FM/TV antenna. During a meteor shower tune your receiver to a distant transmitter between 200 and 1000 miles away. Commercial radio stations, TV stations, and radar transmitters are all suitable if located at the correct distance. Under normal circumstances the transmitter should be difficult or impossible to detect, but when a meteor intervenes the signal hops over the horizon and a brief fragment of the transmission can be heard. Depending on the type of the transmitter it might sound like a tone, a bit of music or voice, or simply noise. Contact lasts for as long as the meteor train persists, usually from 100 milliseconds to a few seconds.

Stan Nelson's echo, above, was obtained at 217 MHz which is usually considered to be a poor frequency for meteor observations. However, the tremendous power of the Naval Space Surveillance radar (NAVSPASUR) more than compensates for its less-than-optimum transmission frequency. NAVSPASUR is an excellent transmitter for meteor observers across the southern United States. For more information about meteor observing with NAVSPASUR, please see the Dec. 1998 Science@NASA article The Ghosts of Fireballs Past. To learn more about radio meteor observing in general, see the North American Meteor Network radio meteor tutorial.

The author wishes to thank Shlomi and Anna Eini as well as members of the North American Meteor Network including Mark Davis, Kim Youmans, Wes Stone and others for their reports of visual meteor sightings on April 22, 1999 during the nominal peak of the Lyrids meteor shower.Web Links